The conventional method of molding by using a compression molding machine as disclosed, for example, in the specifications and drawings of Japanese Laid-open Specification Nos. SHO 60-21225 and SHO 60-110419 of Japanese Patent Applications, respectively, is conducted by charging a plastic resin material to be molded into a heated mold, actuating the molding machine thereby pressuring the material at a low pressure to fill the mold with the resultant molten resin, and then opening the upper and lower molds once slightly to vent the gaseous matter present in the material, pressurizing the material again at a high pressure, continuing the pressurizing and the heating until the material has been cured, and then opening the mold and taking out the molded article. Whilst, in the method of injection and compression molding, when a molten resin material is injected into a mold, the injection flow rate and the injecting speed of the molten resin are controlled by the moving speed and the pressing force of a screw.
In the above-mentioned conventional compression molding method, however, since the resin to be molded is subjected in turn to a low pressure and a high pressure, the number of the processes is increased, and also since after the mold is filled with the resin it is subjected to a predetermined pressure, there is a tendency of molded articles undergoing shrink marks or warping.
Further, in the method of molding by using the injection-compression molding machine, a molten resin is injected into a mold in such a manner that the moving speed and the pressure of the screw is varied with the shape of the mold used and that of an article to be molded. In this case, since the flow speed of the molten resin will change in compliance with the shape of the mold and that of an article to be molded, the mold can not be filled uniformly with the molten resin with the result that the molded articles tend to have defects such as haze weld lines, flow marks and entrap gases.